1. Field of the Invention
The present invention pertains to mining and more particularly to solution mining of soluble minerals.
2. Description of the Related Art
Many valuable minerals are obtained by solution mining of subsurface ores including evaporites. Typically, a cavern is formed by injecting a solvent, which is typically water, and saturating the resulting solution with a desired mineral to the extent possible before bringing it to the surface as a produced brine. The solubility of the desired mineral in the produced brine is a function of temperature, and the underground deposit of the desired mineral is often at a temperature greater than the surface ambient temperature so that a relatively high concentration of the desired mineral is in the produced brine. At the surface, the produced brine is often transported by pipeline to a processing plant, where it is cooled in refrigerated heat exchangers to below ambient temperature in order to cause a portion of the dissolved desired mineral to precipitate due to the reduction in temperature. Alternatively, the temperature of the produced brine may be reduced by evaporation of the solvent, which is typically water, to cause precipitation of solid crystals of the desired mineral. A slurry of the crystals of the desired mineral is processed to recover the crystals, and a depleted brine remains after the crystals are removed. In selective solution mining, the depleted brine can be returned to the source cavern. In non-selective mining, the depleted brine is disposed of as a waste product. Cooling by refrigeration and evaporation is energy intensive and expensive.
U.S. Pat. No. 3,348,883, issued to Jacoby et al. and incorporated by reference, teaches the use of two separate wells drilled into a relatively high temperature mineral deposit, where one of the wells is used for injection and one for production. A warm production brine is produced to the surface, where it is cooled in an evaporative heat exchanger to recover the desired minerals. This is not an optimum process in that evaporation can cause undesired minerals, such as halite, to precipitate, and the thermal energy in the production brine is wasted. With evaporative cooling, very little of the original production brine will remain, and what does remain will be highly contaminated and not suitable for injection into the mineral deposit. In the case of either partial or complete evaporation, a significant quantity of water must be replaced.
U.S. Pat. No. 3,386,768, issued to Jacoby et al. and incorporated by reference, circulates heated water or oil through annuli in a production well to maintain temperature in a production brine in an attempt to prevent salting, which blocks the flow passage in the well due to the deposition of salt in the flow passage. Water or oil is heated in a heat exchanger at the surface and passed downwardly through an annular space in a production well adjacent to a tube through which the production brine flows upwardly, and upon reaching the bottom of the production well, the oil or water returns to the heat exchanger through another annular space.
Another patent, U.S. Pat. No. 5,669,734, issued to Becnel, Jr. et al. and incorporated by reference, described a process for making an underground storage cavern for natural gas in a bedded or domal salt deposit. The '734 patent addressed the problem of accelerating the formation of underground caverns in cold climates by preheating fresh injection water by recovering heat in a produced brine. Halite, which is sodium chloride salt, was solution mined with warm, fresh injection water to increase the rate at which the storage cavern was created, and ambient, cold, fresh water was warmed using a heat exchanger between the cold, fresh water and warm produced brine to provide the warm, fresh injection water. The purpose of the process described in the '734 patent was to make a storage cavern, so there was no discussion of recovering halite from the produced brine, but it would not have been feasible to obtain halite by simply lowering the temperature of the brine because the solubility of halite is only a very weak function of temperature. Heating the fresh injection water increased the rate of dissolution of the halite in the deposit, but did not substantially change the concentration of the halite in the produced brine.
U.S. Pat. No. 3,058,729, issued to Dahms et al. and incorporated by reference, describes a method for solution mining potash, potassium chloride, in which a water solution was injected into a potash deposit and left for months to dissolve the potassium chloride. Brine rich in potassium chloride was produced and conveyed to a shallow cooling pond, where the ambient temperature was relatively cold. Potassium chloride crystals deposited in the pond, and a mother liquor was withdrawn from the pond. A small portion of the mother liquor was purged, and water was added to a large portion of the mother liquor to form the water solution that was fed to the potash deposit. This method requires a cold climate or supplemental means for cooling the produced brine.
Solution mining of potash, potassium chloride, is further described in U.S. Pat. No. 3,918,916, issued to Garrett and incorporated by reference. In the '916 patent, as described with reference to FIG. 6 therein, brine was produced from a potash deposit and initially cooled in a multi-stage vacuum growth-type crystallizer, cooled further in a heat exchange crystallizer that included shell and tube heat exchangers, then cooled further in an atmospheric crystallizing station in which brine flows downwardly over a series of baffles while cold, atmospheric air is drawn upwardly and exhausted by a fan and then optionally, depending on the ambient temperature, cooled further with a refrigerative crystallizer. The produced brine became a slurry containing potassium chloride crystals as it was cooled. The potassium chloride crystals were separated and recovered using physical-separation equipment, such as a cyclone, leaving a brine solution that contained a lower concentration of potassium chloride referred to as a depleted brine. A portion of the depleted brine was recirculated to the shell and tube heat exchangers in the heat exchange crystallizer to cool the produced brine, which warmed the depleted brine. Fresh water was added to the warmed, depleted brine to form a solution that was injected into the potash deposit for dissolving the potassium chloride and forming the produced brine. The method described in the '916 patent requires equipment that is relatively expensive, complex and difficult to maintain and requires a high amount of energy to operate.